Reproductive development in the protandrous gastropod Limax maximus is under photoperiodic control. Male-maturation (gonad enlargement and sperm production) is initiated following a transition from short- (LD 8:16) to long-day (LD 16:8) light cycles. Experiments have indicated that a brain hormone which promotes male-phase gonad growth is secreted under long days. A major objective will be to identify the neurosecretory cells (NSC) that manufacture the hormone so that in later work their electrophysiological characteristics can be examined before and after the transition to long days. Neurosecretory stains will be used to identify NSF and partial-brain implants will be employed to locate brain region(s) involved in production of "maturation hormone". Also, as part of the proposed program the time-course of the developmental response will be characterized by monitoring 3H-thymidine incorporation into gonad DNA after beginning long-days, and the "required number" of long-day cycles for photostimulation of reproductive development will be determined. To initiate the study of photoperiod effects on brain cells, the activity and membrane properties of a group of large phloxinophilic neurosecretory cells recently identified in the right parietal ganglion will be examined in short- and long-day animals. The proposed program will continue one of the first studies of a photoperiodically-regulated molluscan neuroendocrine system controlling sexual development. Given its strong parallels with those mammalian systems in which the neuroendocrine-gonad axis is controlled by photoperiod, the Limax system is potentially an excellent model for studying photostimulated reproductive NSC at the cellular physiology level.